Conventionally, there have been known a number of optical sensor systems for measuring the surface contours and displacements of objects to be measured, and an example of such a system is disclosed in Japanese utility model publication No. 63-7305. Most of such optical sensor systems are based on trigonometric relationships of light beams, and the measuring heads of these systems typically comprise a light projector, a light receiver and a lens system enclosed in a casing of a complicated shape. Therefore, when such a measuring head is mounted on a vibrating object to be measured such as an internal combustion engine, the accuracy of the measurement may be impaired by the vibratory deformation of the measuring head.
Furthermore according to such a sensor system, since the positional relationships of the light projecting system and the light receiving system in relation with the object to be measured are fixed, and the relatively bulky laser generator and photoelectric converters forming such systems have a limited freedom of layout, the overall system tends to be large. For instance, when the thickness of an oil film between the inner wall surface of a cylinder and an outer wall surface of a piston in an internal combustion engine is to be measured during its operation, because of the vibration of the engine, even though it is difficult to solidly attach such a large sensor system to the engine, in reality, a considerable part of the system is required to be mounted directly upon the engine. Therefore, for this reason also, it has been difficult to achieve a high level of accuracy due to the deformation of the measuring system itself.
As an additional consideration, when a plurality of points are to be measured at the same time, one can provide light projecting systems and light receiving systems by the same number as the points to be measured, and can supply laser light from the same number of laser generators to the light projecting systems. However, it creates various problems because not only the size of the overall system increases but also the cost tends to be unacceptably high primarily due to the high cost of the laser generators. It is therefore advantageous to distribute laser light from a common laser generator to each of a plurality of light projecting means, but it is difficult to evenly distribute light to each light projecting means using mirrors and lenses, and the system for distributing light tends to increase the complexity and the size of the overall sensor system.
In order to reduce the influences of the deformation of the measuring system upon the accuracy of dimensional measurement on a vibrating object, it is preferable to use a measuring head having a light projector and a light receiver in a coaxial relationship. This is possible when measuring a gap width by filling the gap with a fluid containing a fluorescent substance which produces fluorescent light when laser light is radiated from the light projector upon it, and measuring the amplitude of the produced fluorescent light with the light receiver.